Ok so i Have not done Physics for about nine years, however here is my problem and from my limited amount of knowledge, my attempt to solve this problem.

Im writing a novel and could really do on checking details with people who have solid Physics background.

If anyone could help me confirm the following I would be grateful.

If an explosion, or an eruption of energy came from the moon controlled or otherwise, would we feel any effects on earth? (Vibrations etc Im assuming based on space having no air that no noise could be heard) Assuming the moon did not shatter or send debris but simply rotated so that the dark side (I know this term is not used to represent the far side of the moon, but im using it here) of the moon can be seen from earth, would anything happen? Again this is assuming the moon did not come closer or further away from the earth, just rotated and then returned back to its usual axis rotation and orbit, the only thing to change is that now we see the far side of the moon at all times instead of the side we have always seen.

As i understand it this angular momentum that will change by the moon will surely have to be matched by the earth (As the two are tidally locked) So i imagine the earth rotation would alter, except i have no idea how it would alter, what would change and if any negative weather patterns or geological upheavals would occur form it. Im sorry my knowledge is so limited but that is the best i can do.

Thanks in advance for the help.

Welcome to PHF!
Ok I'm just a student so I may be wrong in the following: If for "whatever reason" the Moon starts to rotate so that the other face of the Moon is face to face with the Earth, I do not think it would alter the Earth at all. The Moon has a somehow good symmetry, i.e. if you just rotate it and look at its form, you can't really see any difference from its previous position. It's because it's almost spherical so a rotation wouldn't really alter the gravity force the Moon exert on Earth. It would have been different if the Moon had a "chaotic" form like say an asteroid with the same dimensions of the Moon. By chaotic I mean any form that differs a lot from a sphere.
Of course you'd see differences by simply looking at the Moon since craters are different in the case you rotate the Moon, but this has almost no effect.
The big problem in my opinion is how do you rotate the Moon so that you can see its "dark side" from the Earth and then stop it to rotate so that it stays in the same position... It would involve a lot of energy (a torque, that is a tangential force to the surface of the Moon applied and then apply a force in the opposite direction to make is stop to rotate, with respect to Earth) and I'm not sure one can find so much energy without disturbing a lot either the Moon or the Earth or both (this apply if you want a "fast" rotation. The faster you want it, the more energy you'll need to create the torque). That's the real problem in my opinion.

In addition, the Moon does not have to be locked to always show the same face to the Earth. It does it by chance, and if you change it say instantly, then there would be no noticeable gravitational effects. The problem is that by doing so with a "possible" method, you might require a lot of energy coming from somewhere. If you use nuclear bombs or something like that, you might change a lot the form of the Moon and there will be noticeable gravitational effects here on Earth. But I'm not sure in the magnitude of the consequences it would imply.
If you change the orbit of the Moon then you would likely have much greater changes in Earth's one.

__________________
Isaac
If the problem is too hard just let the Universe solve it.

Hey there.

If you do not mind i would like to keep an open dialogue with you? Your the first non judgmental person to reply both with a useful and well detailed explanation, i could really use the help for this novel.
I have posted on so many websites and no one else has provided me with an answer as good as yours (if at all!)

In regards to the energy that would be needed to both begin and stop rotation i was thinking about an internal Engine (assuming of course the moon was hollow and artificial as is the case in my novel.) That would be able to cause a rotation to take place over a period of a few hours, and stop it almost instantaneously.

So just to clarify, if this where to occur, and there was enough energy for this to occur there would be no detrimental effects to the earth?

Thanks again for your help, if you would like to pose any more theoretical questions that might you could foresee as problematic during an event such as the moon rotating in such a manner please don't hesitate do bring it forward.

Elio

How precisely have you intended for this engine to work? My first vision was some sort of set of giant cogwheels rotating the outer shell of the moon, which of course would mean even less of a disturbance, since the motion of the inner part wouldn't change relative to the Earth. Remember Newton's third, action equals reaction, which also means that reaction requires action - if you want the whole rock to rotate, you need to apply an external torque. Like placing two hell-uva-rocket-engines on the equator, on opposite sides and pointing in opposite directions. Of course, chances are you'd just end up tearing the whole thing to pieces. Then again, you talked about it being hollow, so maybe, I dunno, you could connect the engines with the mother of all I beams? Now there's a cool engineering challenge.


It just struck me though - while making the rock itself spin around a little might not make much of a difference, carving out the interior of it sure as hell would play merry hell with things on Earth; the change in gravitational pull would certainly be noticeable.

Yeah i was concerned about the torque having an effect on the moon, but i thought with a beam and a slow rate of turn where both engine were working in union instead of one for rotation, one for stopping rotation, then perhaps it might be ok?
The idea of having two engine pointing in opposite direction and connected by a beam is something i actually had envisioned. They could come out from the hollow core into place, and slowly move the moon into the position i need. (Not sure how slow it would need to be in order not to rip it apart?)
This would provide a spectacle on earth which is what i want however....

The novel is set in a universe where the moon was artificially placed where it is now by an advanced species of alien. It has been hollow from the beginning. It is an artificial moon, so i could play around with the strength of its inner core without to much concern. Because it is an advanced alien built construct Im not sure if propulsion engines would be a bit silly, perhaps something more advanced? Im currently exploring hyperthetical scenarios which if we had the technology to built would work. Like a massive anti gravity device, or if the moon was surrounded in a beam that could rotate it without allowing it to wobble?

Thing is, i want it to have some kind of effect on earth, so that the novel for dramatist purposes, any ideas?
P.S, the turning of the cogs thing is pretty cool, i cannot do that however because the outer layer of crust will be attached to several underground bases, plus it would not be dramatic enough to the people on earth. I need some kind of cool display by the aliens to get peoples attention.

Thanks a lot, at this rate I'm going to put you in the acknowledgments for your scientific help.
Elio

Haha, well, I'm sure none of us who've replied so far would mind that; for my own part I'm just fond of crazy ideas.
You know, I have no idea what the story is, but I'm thinking that if the moon is in fact a giant space station and you want something drastic to happen, then there is a quite reasonable action they might take:
The moon is receding from the Earth at a slow but perceptible rate and has done so for a few billion years, might it not be about time they made a position correction? A sudden change in the mean distance should create some fairly unpleasant tidal effects down here, not to mention the long term effects of tides suddenly sweeping perhaps meters further inland. I'd need to crunch a few numbers to say for certain if it would in fact give any catastrophic results, but maybe that could be something? They just turn on their anti-gravity drive and give it a nudge; down here we suddenly loose the gravitational effects of the moon, only to have it turned on and cranked up five minutes later. Just thinking.

That sounds like an idea, i was thinking of moving the moon a touch closer to compensate for the apparent drift, as like you i thought it would cause a bit of mayhem, but to much. however my concern is:

Someone has written a novel about the catastrophic effects on the earth as the moon came closer, which is annoying because its a cool idea (i do believe however that there' came a hell of alot closer then mine would)

However i like it a lot, a small nudge, you said its been moving away for a billion years, at an inch a year, a billion inches, that's not to close (I imagine you would not see the differences by the naked eye) yet maybe it could have enough effects on earth to cause mayhem.I don't want to wipe out the species or anything, but it does sound like that would wake them up.
So your saying an anti gravity device could help move the moon closer, we would also use this to turn around the moon, then as it is switched off the moons gravitational effects on the earth would kick back into play and cause tidal waves and i imagine strong winds too?

Elio

I think you could move it closer/further by changing its mass. More believable than antigravity might be to transfer material or energy between the moon and some other body in space.

When you said that bringing it closer then switching of the anti gravity device to have the effects of the moon on earth suddenly kick back in, what kind of effects do you think there will be? I have written in winds and waves that wipe out shoreline cities. Decided to use anti gravity because changing the mass would be something simply way over my head, i have a degree in science but its a soft science so all the permutations and calculations would drive me mad!

To transfer mass off the moon you could easily do a bit of E=mc^2 and convert it to radiation that invisibly shoots off into space. As a reader I'd be much more satisfied with something that has a bit of explanation behind it than just a magical 'anti-gravity' that can do whatever the author wants.

You have a valid point. That would be much more satisfying. I will try and see if it fits, but only if you explain it to me! If not then i will try and make the anti gravity understandable by explaining it in detail

Also in this reality (2030) all types of fuel and and explosives have been rendered useless, im figuring out how that happened now, but i need to try and find a way for them to get to the moon without traditional propulsion. My immediate thought was to anti gravity once again, using research i found about how it would theoretically work. If you have any better suggestions let me know.

p.S with the moon coming closer i would still like it to have an effect on the plannet, such as waves and strong winds so hopefully the altering the mass would have this effect still.

Thanks again,
Elio

Actually on second thought maybe changing the mass won't alter its orbit at all. And perhaps the only effect on the earth would be tides being stronger/weaker.

You could use a space elevator to get into space without burning fuel.

By the way, your first post's idea about angular momentum transfer between earth and moon won't apply. If it was rotated by rockets, their exhaust carries the balancing angular momentum. If it's done with a heavy rotating internal part then that's what takes it up.

Ok

Well what I'm going to do is have the moon guided closer to earth by Beams coming from the Pyramids. So just to clarify, if the beams switch of the moons gravity, then switch its effects back on again when the beams are finished, the rush of forces on the earth would cause the devastation such as waves and winds?

Sounds kind of reasonable, but probably only water effects not wind. It would be like the tide changing suddenly instead of over several hours, so the momentum of all that sudden movement should keep the water flowing further inland.

The reason I say not wind is that real tides only go up and down a small distance so that the sea level follows the distorted gravity field. If air moves a similar small distance it wouldn't be much of a wind, not only that but most of the air movement would be in the upper atmosphere, just as most of the water movement is near the surface of the sea.

Hey also, a sudden change in air pressure is an audible sound. Maybe it causes a boom and a destructive shock wave?

Thanks again, especially about the audio sound. Luckily i had written that in already, i guess i knew intuitively! Hey do you know how long it would take the water to recede? I had it coming up to about 200 feet. And the moon coming closer to earth by 15,000 miles.

You might find the next bit fascinating.
The survivors are planning a mission to the moon to see what the hell is going on. Problem is, all types of fuel, including liquid hydrogen, nuclear and explosive elements have been rendered inert by someone who launched something into the upper atmosphere. Now i know not one of those compounds or gasses contains a single variable except oxygen, as hard as it is for scientists to accept its just one of those things that beyond current scientific understanding. So now they have to launch into space without fuel.
Now Im thinking Electirc engines, or an electirc rail gun to launch up into the 45 mile mark or so, then possibly steam to get them over the 62 mark. After then they are going to hope that the fuel will work again.

What do you think?

I have a feeling it ought to be easy to calculate. I think what happens is the sea level tries to reach a shape so that it follows a gravity 'equipotential' or whatever it'd be called. A surface along which gravity has the same strength. Maybe you can just do a bit of Netwonian gravity to find the radius of that surface under the moon vs away from it.


Yea there's already high altitude unmanned electric planes. Or a balloon.

Ion engines can just run on electricity. But it might take more than 2030 technology to make one strong enough to get off the earth.

A gun would put excessive g-force on the astronauts.

An unconservative check on a steam rocket is easy. Find the change in gravitational potential energy between 45miles and 62 miles (Ep = mgh). Then find the amount of water that could hold that much energy when heated from say 0C to 100C. I suspect it will end up impossible, but who knows.

thanks again
i will get on those problems see what i can do.

Just a little curious, how's the novel project coming along?
Since my curiosity works somewhat along the lines of a land mine, I've been reading a little on the subject of anti-gravity lately. Seems like most of those ideas are bound up with the supergravity theories which it in turn seems like just about no one want to pretend they've even heard about these days. Anyway, should you be interested to follow up on the idea this paper and its references might be a good place to start, but yea, it's probably not the best option if you don't want to use the old magic science escape route.

ScienceDirect - Nuclear Physics B : Strong anti-gravity : : Life in the shock wave